Gas bubble likely cause of fish kills

LITTLE ROCK (AGFC) – According to recent tests, two large fish kills below Ozark Dam on the Arkansas River were likely caused by increases in atmospheric oxygen and nitrogen in the water after spillway gates were opened.

About 83,000 freshwater drum and 800 yellow bass died Dec. 29 after 10 spillway gates were open for 10 hours. About 500 drum died Jan. 28 shortly after seven gates were open for two hours.

Bob Limbird, an AGFC district fisheries supervisor; Frank Leone, an AGFC fisheries management biologist; Kelly Winningham, an AGFC fish pathologist, and Andy Goodwin of the University of Arkansas at Pine Bluff conducted experiments Feb. 15 at Dardanelle Dam and Feb. 17 at Ozark Dam and filed the report. They had the cooperation of the U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, UAPB and Arkansas Tech University.

They tested the water below Ozark Dam for temperature, oxygen, oxygen saturation and total gas saturation with spillway gates closed. About 50 minutes after 10 gates were opened, drum and white bass began struggling.

"We collected 21 freshwater drum and four white bass," the report states. "All fish were floating upside down at the water's surface but were still alive. All fish were stiff and had hard sides. Internal examination revealed the fish had distended and full air bladders."

Tests taken after the gates were closed showed the water temperature unchanged, a drop in dissolved oxygen and a rise in total gas saturation.

A similar test below Dardanelle Dam revealed similar results.

According to the report, "The results of our test give us a high level of confidence regarding the cause of death as related to the Dec. 29 and Jan. 28 fish kills observed below the Ozark Dam on the Arkansas River. It is our opinion that these kills were the result of increased atmospheric oxygen and nitrogen gases being added to the water following the opening of the spillway gates of Ozark Dam."

The AGFC Fisheries Division decided to conduct the tests because Winningham and Goodwin believed gas bubble trauma may have caused the Jan. 28 kill. The fish had full air bladders, which made it impossible for them to dive and orient their bodies.

"Freshwater drum are highly susceptible to gas supersaturation and gas bubble trauma," states the report. "In addition, a very deep hole (48 feet deep) exists below gate 5 (Ozark Dam) and may act to congregate freshwater drum during the winter months. These facts may help explain why freshwater drum were the most affected species during the kill events."